This invention relates to a method for making graft block copolymers and particularly to making saturated graft block copolymers without a hydrogenation step. More particularly, the present invention is a process for making saturated graft block copolymers from polyvinyl aromatic hydrocarbons and functionalized polyolefins.
Anionic polymerization utilizing organo alkali metal initiators has long been used to polymerize conjugated diolefins such as butadiene and isoprene and to copolymerize such diolefins with styrenes and substituted styrenes to make styrene-butadiene-styrene and styrene-isoprene-styrene linear block copolymers and similar linear block copolymers. This reaction is usually carried out in an inert hydrocarbon solvent such as cyclohexane or toluene and it is necessary to rigorously exclude oxygen, water or any impurity that can react with a highly reactive propagating species. Under these conditions, the polymeric molecular weights and block structure can be precisely controlled.
Such polymers contain a large amount of aliphatic unsaturation in the diene blocks. Since saturated polymers have certain advantages over unsaturated polymers, such as oxygen and heat stability, it has been desirable to hydrogenate the aliphatic unsaturation in the conjugated diene blocks and this has been accomplished using any of several hydrogenation processes known in the prior art. For instance, the commonly used method is to use a Group VIII metal, particularly nickel or cobalt, catalysts with a suitable reducing agent such as an aluminum alkyl to catalyze the hydrogenation of these polymers. Recently it has been found that bis(cyclopentadienyl)titanium compounds can also be used in the hydrogenation of these polymers. Excellent saturated linear block copolymers are the result of such processes.
The disadvantage in the prior method of producing saturated linear block copolymers is the necessity for the additional hydrogenation and catalyst removal steps. These steps are equipment and time intensive and thereby increase the complexity and cost of producing linear block copolymers. In addition, the hydrogenation catalysts are sensitive to certain poisons making hydrogenation of polymers containing particular functional groups or coupling agent residues difficult or impossible. It would be highly advantageous to have a process by which saturated block copolymers could be produced without the necessity of a hydrogenation step. The present invention provides such a process and also produces graft block copolymers with zero residual unsaturation.
Others have attempted to lithiate EPDM polymers as a first step to the manufacture of saturated grafted polymers (see for instance U.S. Pat. Nos. 4,761,546; 4,786,689; and 4,794,145). These reactions generally involve the metallation of allylic sites in the olefinic moieties of the diene monomer by reaction with alkyl lithium compounds in the presence of activators. These lithiated sites then serve as initiator sites to initiate the polymerization of subsequently added styrene monomer to grow grafted polystyrene blocks.
This method of synthesis does not present a practical process for large scale manufacture of these materials because lithiation efficiencies of only 25 to 35 percent have been achieved. This leaves 65 to 75 percent of unreacted alkyl lithium in solution which, upon addition of styrene monomer, polymerizes the styrene and forms unacceptably large amounts of homopolystyrene, thereby adversely affecting polymer properties. When more forcing conditions are employed to achieve higher lithiation efficiencies, such as higher reaction temperatures, substantial thermal decomposition of the alkyl lithium compounds occurs. An additional disadvantage of this method is that the resulting thermoplastic elastomer contains from about 1 to 4% unsaturation in the rubber backbone due to diene comonomer. These unsaturated sites are prone to degradation in the presence of heat and/or chemicals resulting in loss of material properties. The present invention provides a different method of producing the desired materials which does not have these problems.